Chapter 14 - Examples of CPU’S
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Chapter 14 - Examples of CPU’s In this chapter I will briefly describe the important CPU’s which have been on the market, starting from the PC’s early childhood and up until today. One could argue that the obsolete and discontinued models no longer have any practical significance. This is true to some extent; but the old processors form part of the “family tree”, and there are still legacies from their architectures in our modern CPU’s, because the development has been evolutionary. Each new processor extended and built “on top of” an existing architecture. Fig. 98. The evolutionary development spirals ever outwards. There is therefore value (one way or another) in knowing about the development from one generation of CPU’s to the next. If nothing else, it may give us a feeling for what we can expect from the future. 16 bits – the 8086, 8088 and 80286 The first PC’s were 16-bit machines. This meant that they could basically only work with text. They were tied to DOS, and could normally only manage one program at a time. But the original 8086 processor was still “too good” to be used in standard office PC’s. The Intel 8088 discount model was therefore introduced, in which the bus between the CPU and RAM was halved in width (to 8 bits), making production of the motherboard much cheaper. 8088 machines typically had 256 KB, 512 KB or 1 MB of RAM. But that was adequate for the programs at the time. The Intel 80286 (from 1984) was the first step towards faster and more powerful CPU’s. The 286 was much more efficient; it simply performed much more work per clock tick than the 8086/8088 did. A new feature was also the 32 bit protected mode – a new way of working which made the processor much more efficient than under real mode , which the 8086/8088 processor forced programs to work in: http://www.karbosguide.com/books/pcarchitecture/chapter15.htm 1 • Access to all system memory – even beyond the 1MB limit which applied to real mode. Access to multitasking , which means that the operating system can run several programs at the same time. • The possibility of virtual memory , which means that the hard disk can be used to emulate extra RAM, when necessary, via a swap file . • 32 bit access to RAM and 32 bit drivers for I/O devices. Protected mode paved the way for the change from DOS to Windows, which only came in the 1990’s. Fig. 99. Bottom: an Intel 8086, the first 16-bit processor. Top: the incredibly popular 8-bit processor, the Zilog Z80, which the 8086 and its successors out competed. 32 bits – the 80386 and 486 The Intel 80386 was the first 32-bit CPU. The 386 has 32-bit long registers and a 32-bit data bus, both internally and externally. But for a traditional DOS based PC, it didn’t bring about any great revolution. A good 286 ran nearly as fast as the first 386’s – under DOS anyway, since it doesn’t exploit the 32-bit architecture. The 80386SX became the most popular chip – a discount edition of the 386DX. The SX had a 16- bit external data bus (as opposed to the DX’s 32-bit bus), and that made it possible to build cheap PC’s. Fig. 100. Discount prices in October 1990 – but only with a b/w monitor. The fourth generation The fourth generation of Intel’s CPU’s was called the 80486. It featured a better implementation of the x86 instructions – which executed faster, in a more RISC-like manner. The 486 was also the first CPU with built-in L1 cache. The result was that the 486 worked roughly twice as fast as its predecessor – for the same clock frequency. With the 80486 we gained a built-in FPU. Then Intel did a marketing trick of the type we would be better off without. In order to be able to market a cheap edition of the 486, they hit on the idea of disabling the FPU function in some of the chips. These were then sold under the name, 80486SX. It was ridiculous – the processors had a built-in FPU; it had just been switched off in order to be able to segment the market . http://www.karbosguide.com/books/pcarchitecture/chapter15.htm 2 Fig. 101. Two 486’s from two different manufacturers. But the 486 was a good processor, and it had a long life under DOS, Windows 3.11 and Windows 95. New editions were released with higher clock frequencies, as they hit on the idea of doubling the internal clock frequency in relation to the external (see the discussion later in the guide). These double-clocked processors were given the name, 80486DX2. A very popular model in this series had an external clock frequency of 33 MHz (in relation to RAM), while working at 66MHz internally. This principle (double-clocking ) has been employed in one way or another in all later generations of CPU’s. AMD, IBM, Texas Instruments and Cyrix also produced a number of 80486 compatible CPU’s. Pentium In 1993 came the big change to a new architecture. Intel’s Pentium was the first fifth-generation CPU. As with the earlier jumps to the next generation, the first versions weren’t especially fast. This was particularly true of the very first Pentium 60 MHz, which ran on 5 volts. They got burning hot – people said you could fry an egg on them. But the Pentium quickly benefited from new process technology, and by using clock doubling, the clock frequencies soon skyrocketed. Basically, the major innovation was a superscalar architecture. This meant that the Pentium could process several instructions at the same time (using several pipelines). At the same time, the RAM bus width was increased from 32 to 64 bits. Fig. 102. The Pentium processor could be viewed as two 80486’s built into one chip. Throughout the 1990’s, AMD gained attention with its K5 and K6 processors, which were basically cheap (and fairly poor) copies of the Pentium. It wasn’t until the K6-2 (which included the very http://www.karbosguide.com/books/pcarchitecture/chapter15.htm 3 successful 3DNow! extensions), that AMD showed the signs of independence which have since led to excellent processors like the AthlonXP. Fig. <![endif]-- >103. One of the earlier AMD processors. Today you’d hesitate to trust it to run a coffee machine… In 1997, the Pentium MMX followed (with the model name P55), introducing the MMX instructions already mentioned. At the same time, the L1 cache was doubled and the clock frequency was raised. Fig. 104. The Pentium MMX. On the left, the die can be seen in the middle. Pentium II with new cache After the Pentium came the Pentium II. But Intel had already launched the Pentium Pro in 1995, which was the first CPU in the 6 th generation. The Pentium Pro was primarily used in servers, but its architecture was re-used in the popular Pentium II, Celeron and Pentium III models, during 1997-2001. The Pentium II initially represented a technological step backwards. The Pentium Pro used an integrated L2 cache. That was very advanced at the time, but Intel chose to place the cache outside the actual Pentium II chip, to make production cheaper. http://www.karbosguide.com/books/pcarchitecture/chapter15.htm 4 Fig. 105. L2 cache running at half CPU speed in the Pentium II. The Level 2 cache was placed beside the CPU on a circuit board, an SEC module (e.g. see Error! Reference source not found. http://www.karbosguide.com/books/pcarchitecture/chapter15.htm 5 Chapter 15. Evolution of the Pentium 4 As was mentioned earlier, the older P6 architecture was released back in 1995. Up to 2002, the Pentium III processors were sold alongside the Pentium 4. That means, in practise, that Intel’s sixth CPU generation has lasted 7 years. Similarly, we may expect this seventh generation Pentium 4 to dominate the market for a number of years. The processors may still be called Pentium 4, but it comes in al lot varietes. A mayor modification comes with the version using 0.65 micron process technology. It will open for higher clock frequencies, but there will also be a number of other improvements. Hyper-Threading Technology is a very exciting structure, which can be briefly outlined as follows: In order to exploit the powerful pipeline in the Pentium 4, it has been permitted to process two threads at the same time . Threads are series of software instructions. Normal processors can only process one thread at a time. In servers, where several processors are installed in the same motherboard (MP systems), several threads can be processed at the same time. However, this requires that the programs be set up to exploit the MP system, as discussed on page 31. The new thing is that a single Pentium 4 logically can function as if there physically were two processors in the pc. The processor core (with its long pipelines) is simply so powerful that it can, in many cases, act as two processors. It’s a bit like one person being able to carry on two independent telephone conversations at the same time. Figur 110. The Pentium 4 is ready for MP functions. Hyper-Threading works very well in Intel’s Prescott-versions of Pentium 4. You gain performance when you operate more than one task at the time. If you have two programs working simultaneously, both putting heavy pressure on the CPU, you will benefit from this technology.